This invention relates to constant velocity ratio universal joints of the tripod type. A joint of the type to which the invention relates comprises an outer joint member having a rotational axis and three guide grooves extending parallel to its rotational axis and equally circumferentially spaced thereabout; an inner joint member disposed inside the outer member, having a rotational axis and three arms equally spaced about this rotational axis extending radially into the guide grooves of the outer joint member; each arm carrying a roller having an external surface which engages opposed side portions of the corresponding guide groove so that the roller is constrained to roll therealong; each roller being able to rotate about, move lengthwise of, and tilt relative to the arm by which it is carried. Such a joint will hereafter be referred to as a tripod joint of the kind specified.
In a tripod joint of the kind specified, the constraint of each roller to rolling movement along its respective guide groove in the outer joint member, without tilting relative thereto, means that the joint has a reduced frictional resistance to plunge (i.e. relative axial movement between the outer and inner joint members) and rotation when the joint is articulated. When the joint rotates in the articulated condition, each roller tilts relative to the arm by which it is carried rather than relative to the groove in which it is engaged. It will be appreciated that if the roller tilted relative to the groove, it would not be able to roll along the groove but would have to slide therealong in the tilted condition, which obviously would produce a greater frictional resistance to such movement.
There have been various proposals for arrangements by which each roller is carried by its respective arm so as to be able to undergo the necessary rotational, sliding and tilting movement relative thereto. For example, in U.S. Pat. No. 4,379,706 there is disclosed a tripod joint of the kind specified wherein each arm has an outer cylindrical surface and each roller has an inner cylindrical surface, between these two surfaces there being disposed inner and outer guide rings which have interengaging part-spherical surfaces. A needle roller bearing is disposed between the outer surface of the outer guide ring and the internal cylindrical surface of the roller. In this arrangement, the roller is able to tilt relative to the arm by virtue of the interengaging part-spherical surfaces of the inner and outer guide rings. The inner guide ring is able to slide lengthways on the arm. The roller is able to rotate about the arm by virtue of the needle roller bearing between the outside of the outer guide ring and the inner cylindrical surface of the roller.
A further example of a tripod joint of the kind specified is disclosed in JP-UM Laid Open No. 63-57822. In this joint, each arm of the inner joint member has a cylindrical surface and each roller comprises inner and outer roller elements with inter-engaging part-spherical surfaces. A needle roller bearing assembly is disposed between the internal surface of the inner roller element and the cylindrical surface of the arm. The outer roller element is able to tilt relative to the arm by virtue of the interengaging part-spherical surfaces of the inner and outer roller elements, while both the inner and outer roller elements together are able to rotate about the arm and slide lengthways of the arm, such rotational and sliding movement together taking place at the needle roller bearing between the internal surface of the inner roller element and the outer cylindrical surface of the arm.
In JP-UM 63-57822, the outer peripheral surface of the outer roller element is toroidal, i.e. it is a surface of rotation, about the axis of the roller, of an arc which, in a section through the roller element viewed lengthwise of the guide groove in the outer joint member, has a radius of curvature smaller than the outer diameter of the outer roller element. Each side portion of the guide groove, where it is engaged by the outer roller element is of "gothic arch" section, comprising two arcuate portions with different centres of curvature so that the outer peripheral surface of the outer roller element contacts the groove side portion at two points (although it will be appreciated that when under load the roller/groove engagement is through small elliptical areas of contact rather than at true points). This condition, referred to as angular contact, provides for a reduction of friction between the outer roller element and groove when rolling therealong, and also such a configuration resists tilting of the outer roller element within the groove.
In practice the effectiveness of the angular contact between roller and groove in preventing the roller from tilting is subject to tolerances which inevitably are present when these components are mass produced. If the roller element fitted in the groove with no backlash (clearance) whatsoever, then it would be impossible for the roller element to tilt within the groove. If, however, as will in practice be the case, the roller element is not a perfect fit in the groove and there is some clearance or backlash therebetween, the roller element will be able to tilt within the groove to a small extent. This is particularly a problem when the joint is transmitting little or no torque, when the roller element is free to tilt until the clearance between it and the groove is taken up. When the joint is transmitting torque, the fact that the roller element is urged into engagement with a side portion of the groove has the effect that the roller element is brought into alignment with the groove. The geometry of the surfaces of the roller element and groove side portion, engaging with angular contact, is such that any tendency of the roller element to tilt produces a couple which acts on the roller element to tend to restore it to its aligned orientation.
Since the smoothness of operation of a joint of the kind specified, however, is dependent on the maintenance of the roller elements in or as close as possible to the correct alignment in the grooves under all conditions of joint operation, it is recognised that if the control of the alignment of the roller elements can be improved then joint performance can be enhanced. It is, accordingly, the object of the present invention to provide a tripod joint of the kind specified wherein such improved control of the alignment of the roller elements can be achieved.